The prevalence of penicillin-resistant pneumococci (PRP) has been increasing, with the highest levels reported from countries with relatively unrestricted antimicrobial use. It has been low in northern Europe except Iceland, which is disconcerting as antimicrobial use in Iceland has been relatively restricted. This suggests that other factors may facilitate their spread. By studying their epidemiology and possible risk factors for carriage, we have attempted to explain their rapid spread in Iceland. The incidence of infections caused by PRP (as percentage of infections considered due to pneumococci) has increased from 0% in 1988 to 2.3% in 1989, 2.7% in 1990, 8.4% in 1991, 16.3% in 1992, and 19.8% in 1993. The infections have mainly affected 0- to 3-year-old children (71.4%), and the PRP belonged almost exclusively to serogroups 6, 19, and 23 (98.8%). Most were serotype 6B multiresistant (75%; resistant to penicillin (MIC = 1.0), cephalothin, erythromycin, clindamycin, tetracycline, chloramphenicol, fusidic acid, sulfonamides, and trimethoprim), and believed to belong to a single clone originating from Spain. The PRP have been prevalent in healthy children, 0-10% nasopharyngeal carriage, especially in day-care centers, with the highest prevalence in areas that had the highest antimicrobial consumption. Recent antimicrobial consumption, especially of trimethoprim-sulfa, appeared to increase PRP carriage. The rapid spread of PRP in Iceland may have been facilitated by high antimicrobial consumption in day-care centers (especially of trimethoprim-sulfa) which are attended by the majority of Icelandic children.